The design and operation of single-loop, countercurrent spray towers utilizing limestone is discussed by Rader and Bakke, in Incorporating Full-Scale Experience Into Advanced Limestone Wet FGD Designs, presented at the IGCI* Forum 91, Sep. 12, 1991, Washington, D.C. (*formerly the Industrial Gas Cleaning Institute, now the Institute of Clean Air Companies, Washington, D.C.). Open spray towers (i.e., those not having packings, trays or other means for facilitating gas-liquid contact) are simple in design and provide high reliability. They are especially useful in coal-fired power stations where the evolution of chlorides has caused a number of problems, including reduced reactivity of the scrubbing solution and severe corrosion of scrubber internals. Another factor favoring the use of open spray towers is their inherent low pressure loss and resulting fan power economy.
In single-loop, countercurrent, open scrubbing towers of the type discussed by Rader and Bakke, a scrubbing slurry composed of calcium carbonate, calcium sulfate, calcium sulfite, and other non-reacting solids flows downwardly while the SO.sub.x -laden effluent gas flows upwardly. The SO.sub.x, principally S02, is absorbed in the descending scrubbing slurry and is collected in a reaction tank where calcium sulfite and calcium sulfate are formed. Desirably, the reaction tank is oxygenated to force the production of sulfate over sulfite. Once the crystals of sulfate are grown to a sufficient size, they are removed from the reaction tank and separated from the slurry. This reference and the one that follows are incorporated by reference in their entireties for the descriptions of the known technology of wet scrubbing.
Rader and Bakke note that pumping costs account for about 50% of the total power requirements of the system. They show spray levels oriented at acute angles relative to each other and suggest spacing between the levels of from 5 to 6 feet. The spacing is compelled by what is considered in the prior art to be appropriate for constraints of the header and support systems. With spacings between the angularly-oriented spray levels at the suggested values, tower height and pumping costs will increase. With lower spacings, servicing will become extremely difficult.
In U.S. Pat. No. 5,173,093, Johnson, et al., describe a single-spray level, which is designed to take the place of multiple spray levels, in a flue gas desulfurization system. The arrangement described has a plurality of spray headers, all arranged in parallel within a common horizontal plane. The device, having all spray heads in one plane, cannot provide for efficient overlap of spray patterns to successfully deal with the problem of gas pass through or channeling.
The prior art does not address the points necessary to reduce tower height for wet scrubbers for SO.sub.x reduction, while addressing the issue of servicing in an efficient manner.